Method for producing a surface layer of high brightness on lignocellulosecontaining material



FIPSSU? 3 173 748 METHOD FOR PRODUCING A SURFACE LAYER OF HIGHBRIGHTNESS N LIGNOCELLULOSE- CONTAINING MATERIAL Ernst Ludvig Back,Lidingo, Siiren Ake Ivan Eriksson, Stockholm, and Leif Olof Klinga,Akersberga, Sweden, assignors to Stiftelsen WallboardindustriensForskningsinstitut, Stockholm, Sweden, a joint-stock company of SwedenNo Drawing. Filed June 25, 1962, Ser. No. 205,069

12 Claims. (Cl. 8-107) Several methods have been proposed to producefrom lignocellulose containing fibres boards and other thick sheetmaterial with high brightness. Accord-ing to one of the said methods,the wet web of the material during its production is provided with oneor several surface layers of fibrous material having a brightnessconsiderably exceeding the brightness of the underlaying main web layer.The medium used for said surface layer or layers is usually a fibrousmaterial pre-treated in one way or another, for example a bleachedfibrous material, the thickness of the bleached surface layer beingchosen to be such that it provides a satisfactory covering effect. Theaforesaid method is predominantly used, for example, in the manufactureof card-board, boxboard, paper-board and fibre building board. As analternative, a surface layer of non-fibrous material may be applied, forexample, a bright pigment with a binding agent.

Non-flat, multiformed products, usually called moulded pulp products arewet-formed for example by vacuumdewatering of a pulp stock and usuallyin a single step, whereafter the partly dewatered, ready-formed productis dried in hot air. Such moulded pulp products are egg cartons, fruittrays, rough-trays, flower containers etc. They usually are producedfrom groundwood or from waste paper stock or from mixtures of these two.Methods of wet-forming a surface layer of bleached pulp on one or theother side of such moulded pulp product so far are complicated andexpensive.

It has now been found, according to the present invention, thatlignocellulose containing materials of desirable thickness and highbrightness can be produced in a simple as well as a technically andeconomically advantageous manner by applying to the ready-dried paperboard or moulded pulp product a bleaching agent composed entirely orsubstantially of reducing bleaching agents, without subsequentlyremoving degradation products of the bleaching reaction. The bleachingagent may be applied to the surface in the form of an aqueous solutionimmediately after or in connection with the drying of the material andprior to cutting or dividing it up into units for use.

In many cases it is preferred that only one major surface of thematerial to be brightened is treated with the bleaching agent.Illustrative of such materials are moulded egg cartons, corrugatedcontainer board and corrugated container blanks being treated in advanceof printing. Also the treatment may be applied to a part of the materialsurface, e.g. a part forming a single face of a corrugated container.

As bleaching agents showing good results mention may be made ofhydrosulphite, such as sodium or zinc hydrosulphite (dithionite),possibly with an addition of ethylenediamine tetrasodium acetate, andsodium or potassiumborohydride, and also combinations of the same insuch a manner that, for example, first sodium hydrosulphite is appliedand thereafter sodiumborohydride.

The bleaching agent is preferably applied to a dry and heated web of thematerial as it leaves a dryer. It has been found that, for obtainingmaximum bleaching effect, the amount of aqueous solvent to be applied tothe web together with a certain amount of bleaching agent, must beadjusted in accordance with the temperature of the web. Highertemperatures have proved to require a greater amount of aqueous solvent.It is essential that the bleaching agent be applied in an amount thatensures a thin but fully covering bleached layer.

The maximum bleaching efiect depends on the bleaching reaction, thenecessary bleaching time at different temperature in view of thereaction speed, the surface temperature and the surface pH of the web,as well as on the depth to which the bleaching agent may act andpenetrate in view of the temperature, the water amount added and theporosity of the material.

The bleaching agent may be sprayed on the material in conventionalmanner by means of fine nozzles using high water pressure or an airstream, but may under certain conditions even be applied together withthe wetting water in the combined reconditioning and cooling of theproduct mat.

The material to be treated is usually paperboard, cardboard or boxboardespecially of high basis weight, e.g. at least 200 gms. per square metreand preferably exceeding 500 gms. per square metre. It may be kraftliner. Valuable as material are also a multiform moulded pulp productmanufactured of a substance comprising lignocellulose containing fibres.Illustrative of such a product are egg cartons, fruit trays, roughtrays, plates and flower containers. The material may be made upentirely or partially of groundwood or krafit pulp. If desired, it mayalso contain or completely be made up of Waste paper stock.

The following examples are illustrative of the invention. It is to beappreciated that the materials and the manufacturing conditions setforth are exemplary and the invention should not be considered to belimited thereto. The brightness is indicated as dilfused reflection oflight of 4570 A. wave length in percent of magnesium oxide reflectionmeasured with the General Electric brightness meter on flat pieces ofthe material.

Example 1 A thoroughly dried web of wet formed trays of groundwood witha basis weight of 400 g./m. when leaving the dryer with a surfacetemperature of C., was sprayed with 200 ml./m. of 8% sodiumhydrosulphite solution (Na S O The solution had the adjusted pH-value6.5 and was mixed with 3% of EDTA (ethylenediamine tetrasodium acetate)calculated on the basis of the Na S O content. After the spraying, thematerial was worked in conventional manner. As a result of thistreatment, the brightness of the product increased from 53% to 62%. Thebrightness stability was measured in a Fade-o-meter whereby thebrightness after 10 hours light treatment decreased from 62% to 56%,while the brightness of the untreated sheet decreased from 53% to 44% Inanother test the surface temperature of the dry sheet was reduced bycold air blowing to 49 C. Whereafter the sheet was sprayed with mL/m. ofa 3% zinc hydrosulphite solution. The brightness increased from 53% to60% and decreased after Fade-o-meter treatment for 10 hours to 50%.

Example 2 Wet formed moulded egg cartons with a basis weight of 385 gms.per square metre and a projected basis weight of 730 gms. per squaremetre and with a surface temperature of 80 C. were sprayed first with 50nil/m. of a 20% sodium hydrosulphite solution and thereafter with 50mL/m. of a 0.5% sodiumborohydride solution. By

this treatment, the brightness of the material was increased from 60.3%to 65.2%.

If instead of the dry web was sprayed with 50 nil/m. of a 1.0% solutionof sodiumborohydride the brightness of the material increased from 60.3%to 68.8%.

Example 3 A moulded egg carton of groundwood and waste. newspaper stockwith a basis weight of 330 g./m. was sprayed with 100 ml./sq. m. of a1.0% water solution of sodium borohydride. The brightness of the productincreased from 40% to 46% while as well the yellow colour of the productessentially into a more pleasant grey colour.

Example 4 A triplex groundwood card-board with a weight of 1200 gms. persquare metre was sprayed with 100 ml./m. of a 0.2% sodiumborohydridesolution. The spraying was carried out in the form of atomized sprayapplied to the running web after its drying. By this treatment, thebrightness was increased from 53% to 59%. After light treatment in aFade-o-meter for 10 hours the brightness decreased from 59% to 55%,while the brightness of the untreated card-board decreased from 53% to50%.

Example 5 A high yield unbleached kraftliner with a basis weight of 215gms./ sq. m. was sprayed at room temperature with 30 ml./ sq. m. of a20% solution of sodium hydrosulphite (sodium dithionite) containing also0.3% of ethylendiaminotetrasodium acetate the reflectance of this sheetat 4570 A. increased from 18.0% to 20.7%. The reflectance at 5500 A.increased from 28.0% to 34.2%.

If instead the sheet was sprayed with 50 rnl./m. of a 2% solution ofsodium borohydride the reflectance of 4570 A. increased from 18.1 to23.4% and the reflectance at 5500 A. from 27.9% to 37.1%.

Spraying the same solutions on one side of a ready made corrugator boxgave essentially similar results.

The invention in its broader aspects is not limited to the specificprocesses, materials, bleaching agents, combinations and improvementsdescribed but departures may be made therefrom within the scope of theaccompanying claims without departing from the principles of theinvention and without sacrificing its chief advantages.

What we claim is:

1. A method of providing pre-formed, dried lignocellulose-containingmaterial with only a surface layer of high brightness comprisingapplying a bleaching agent consisting essentially of reducing bleachingagents to said surface of the pre-formed' and driedlignocellulosecontaining material in an amount Sufficient to react withonly a surface layer of the latter, and permitting the reaction productsto remain in the surface layer of the material.

2. A method as in claim 1; wherein said bleaching agent is applied in anamount of water small enough to be absorbed by the material withoutrequiring further drying of the latter.

3. A method as in claim 2; wherein said bleaching agent and water areapplied immediately after drying of the lignocellulose-containingmaterial.

4. A method as in claim 1; wherein said bleaching agent is sodiumhydrosulphite.

5. A method as in claim 4; wherein said sodium hydrosulphite has anaddition thereto of ethylenediamine tetrasodium acetate.

6. A method as in claim 1; wherein said bleaching agent is zinchydrosulphite.

7. A method as in claim 6; wherein said zinc hydrosulphite has anaddition thereto of ethylenediamine tetrasodium acetate.

8. A method as in claim 1; wherein said bleaching agent is analkaliborohydride.

9. A method as in claim 1; wherein said lignocellulosecontainingmaterial is made up, at least in part, from ground wood pulp.

10. A method as in claim 1; wherein said lignocelluosecontainingmaterial is made up, at least in part, from unbleached kraft pulp. V

11. A method as in claim 1; wherein said material is of sheet-like formand has a basic weight of at least 200 gms. per square meter.

12. A product prepared by the method according to claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,707,145 8/55Sparrow et al. 16271 FOREIGN PATENTS 138,290 8/50 Australia.

846,079 8/60 Great Britain.

610,654 12/60 Canada.

611,510 12/60 Canada. 1,103,121 3/61 Germany.

625,011 8/61 Canada.

OTHER REFERENCES Du Pont: The Technical Bulletin, September 1955, vol11, No. 3, pages 136-138.

NORMAN G. TORCHIN, Primary Examiner.

1. A METHOD OF PROVIDING PRE-FORMED, DRIED LIGNOCELLULOSE-CONTAININGMATERIAL WITH ONLY A SURFACE LAYER OF HIGH BRIGHTNESS COMPRISINGAPPLYING A BLEACHING AGENT CONSISTING ESSENTIALLY OF REDUCING BLEACHINGAGENTS TO SAID SURFACE OF THE PRE-FORMED AND DRIEDLIGNOCELLULOSECONTAINING MATERIAL IN AN AMOUNT SUFFICIENT TO REACT WITHONLY A SURFACE LAYER OF THE LATTER, AND PERMITTING THE REACTION PRODUCTSTO REMAIN IN THE SURFACE LAYER OF THE MATERIAL.